This invention relates to a boron-containing, austenitic stainless steel alloy and in particular to such an alloy and an article made therefrom which in addition to neutron absorption and corrosion resistance, has a unique combination of tensile ductility, strength and toughness such that the alloy is especially suited for making load-bearing, structural members.
Heretofore, an alloy of AISI type 304 stainless steel containing about 0.08% max. carbon, 2.00% max. manganese, 1.00% max. silicon, 0.045% max. phosphorus, 0.03% max. sulfur, 18.0-20.0% chromium, 8.0-10.5% nickel, 2.0% max. boron, and the balance essentially iron has been employed for making articles used in the nuclear power industry because of the good neutron absorption and corrosion resistance which the alloy provides. Here and throughout the specification and claims percent (%) will mean percent by weight unless otherwise specified. It is known that 1-2% boron benefits the tensile strength of the alloy, but also impairs the tensile ductility and toughness of the alloy. The presence of not more than about 1% boron in the alloy provides satisfactory ductility, but with a substantial sacrifice in neutron absorption capability. Accordingly, because of insufficient impact toughness and tensile ductility when the alloy contains more than 1% boron, it has not been found suitable for use in structural members or other load-bearing articles There has been a growing need for an alloy combining high impact toughness together with good corrosion resistance, strength and high neutron absorption for use as a structural material.
The present invention stems from the discovery that in the alloy of this invention, at each level of boron content the boron is present as complex borides, usually but not necessarily in the form of M.sub.2 B type borides, and that when the quantity and distribution of the borides are controlled, as will be more fully described hereinbelow, a hitherto unattainable and unique combination of high strength, ductility, impact toughness and thermal neutron absorption at each level of boron is consistently obtained. Heretofore, it had been found that increasing boron in excess of 0.2% to improve neutron absorption was characterized by increases in difficulty and randomness with the result that a significant proportion of the material produced had marginal or less than desired mechanical properties. Thus, an important feature of the present invention results from the discovery that for a given level of boron in the alloy there is a readily determinable minimum normalized a real density (A.sub.N) of the borides present therein which is characteristic of articles produced from the alloy and which ensures consistent attainment of the unique combination of neutron absorption, strength, ductility, impact toughness and corrosion resistance characteristic of the present invention.
It is a principal object of this invention to provide an alloy and article made therefrom having a unique and desirable combination of neutron absorption, corrosion resistance, tensile ductility, strength and impact toughness.